Owing to the recent remarkable development in the art of laser, it has become possible to easily obtain miniature devices of high-output solid laser and semiconductor laser which have the emission region into the near infrared or infrared region. Such the laser devices are very useful as exposure sources upon direct plate-making from digital data of a computer or the like.
Conventionally, as a planographic printing plate of the type which enables direct drawing thereon from digital data of a computer, an electrophotographic system utilized-printing plate, a printing plate of the photopolymerization type which is exposed with argon or YAG laser beams and, if needed, post-heating in combination therewith, a printing plate laminating on a photosensitive resin layer a silver salt photosensitive material, a printing plate of the silver salt diffusion transfer type, and a printing plate of the type which a silicone rubber layer destroyed by an electric discharge or exposure to laser beams are known.
The electrophotographic system-utilized printing plate involves complicated processing operations, namely charging, exposure and development process, and so a large-sized complex apparatus is required. The printing plate of the photopolymerization type essentially has a sensitivity limit, and so it is difficult to apply a miniature laser device thereto and to handle it in a bright room. The printing plate having a layer of silver salt photosensitive material and the printing plate of the silver salt diffusion transfer type are complicated in processing operations and have high cost. The printing plate of the type which has a silicone rubber layer destructible by laser beams has a problem of removal of silicone sediment remaining on the plate surface.
As the art of enabling the image formation with those light sources, JP-A-52-113219 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses the positive working recording material which comprises a compound decomposable by light and heat (e.g., a diazonium compound), particles of a material capable of absorbing light and transforming the absorbed light into heat, and a binder. In addition, JP-A-58-148792 discloses the light- and heat-sensitive recording material which contains as main components particles of a thermoplastic resin, a light-to-heat transformation material and a photo-crosslinking material (e.g., diazonium compound).
However, the above-described recording materials which are usable laser beams as a light source have low sensitivity. In case of a direct plate-making system, the above-described positive working materials require a considerably long writing time since digital data of a computer or the like are directly written on the plate material by scanning the laser beams. With the intention of reducing the writing time, therefore, it has been expected to develop a highly sensitive material (if possible, of the negative working type) which enables the writing with a solid or semiconductor laser (heat mode) device having emission in the near infrared or infrared region.
Such being the case, conventional planographic printing plates which enable direct drawing from digital data of a computer were insufficient.
On the other hand, a vinyl ether group-containing compound is generally known as a species of polymerizing compound. The vinyl ether group-containing compound have been mainly developed as a cationic polymerizable compound. The cationic polymerization has characteristics, e.g., in that monomers can undergo polymerization without inhibition by oxygen and a contraction in volume is small on polymerization, as compared with radical polymerization. Accordingly, the combination of a vinyl ether group-containing compound with a photoacid generator, which can provide a UV curable resin, has been used for ink and coating materials, recently for the formation of a three dimensional pattern.
Examples of the compounds known as a photoacid generating agent (photocationic polymerization initiator) include diazonium salts, sulfonium salts, iodonium salts, pyrylium salts, thiopyrylium salts and so on. All the combinations of the aforesaid monomers for cationic polymerization with those photoacid generating agents are insufficient in sensitivity. On the other hand, photocationic polymerizable monomers having high sensitivity are described in Stephen C. Lapin, Polymers Paint Colour Journal, 179 (4237), 321 (1989).
Further, when the aforesaid monomers are combined, similarly to the above, with photoacid generating agents and the acid-hydrolyzable property of vinyl ether groups present therein is utilized, the resulting combinations can be used as positive working materials of the chemical amplification type, as recently disclosed in JP-A-06-230574 and JP-A-62-45971.
However, only exposure-light sources which can emit light of wavelengths of 450 nm or less can be employed since most of the photoacid generating agents, which are practically effective when used in the aforementioned art, absorb light at wavelengths of 450 nm or less.